It is an unfortunate fact that terrorists often attempt to influence the course of political events through the use of violence. One infamous means of implementing these violent actions, such as the 1988 terrorist bombing of a Pan American flight over Lockerbie, Scotland, is by strategically placing bombs where they will cause the greatest devastation and have the greatest political impact. Indeed, bombs almost seem to be a terrorist weapon of choice. As is well known, terrorist targets are typically chosen on the basis of their vulnerability to such attack and are frequently, if not purposefully, selected without regard for human life.
Crowds of people can, therefore, be an attractive terrorist target due to the intense public reaction that mass murder provides. Further, vehicles are attractive targets because they are compact and will almost always contain people when they are being operated. Terrorists also target containers, such as trash containers and mailboxes located in crowded areas. Aircraft effectively combine these attractions for terrorists.
Despite extremely high security procedures and the use of sophisticated explosive detecting equipment, bombs have still occasionally found their way aboard aircraft. Typically, bombs have been hidden in passenger luggage or in parcels that are stored and carried in the cargo compartment of an aircraft. Since there is a limit to the size of bomb that can be relatively easily detected, one security strategy for protecting the aircraft against the internal detonation of an explosive device is to recognize that small explosive devices may not always be detected, and then plan on ways to reduce the damage which can be caused by such a device.
In the airline industry, it is standard practice to compartmentalize the cargo that is to be carried on board the larger aircraft. This is effected by separating the cargo into separate units, and placing these units of cargo into individual containers, which are commonly referred to as unit load devices (ULDs). Because of regulatory requirements, as well as practical considerations, the shape, size and weight of a ULD for each type of aircraft has been standardized. Consequently, in order to design a ULD that will meet the standard requirements of the industry, and still effectively withstand a substantially large blast from an explosion in the cargo hold within the ULD, the limitations of cost, size and mass must be considered.
Typically, ULDs are shaped as boxes that can include appropriately sloped surfaces that conform the ULD to the aircraft's fuselage when the ULD is in place in the aircraft's cargo compartment.
Essentially, the container is made of several panels which are joined together to form the ULD. Additionally, each ULD has a door or an access hatch, which allows it to be opened for placing cargo into the ULD or for removing cargo from the ULD.
ULDs have typically been made of aluminum, which is light in weight but is not explosion proof. As a consequence, there has been tremendous focus in recent years on redesigning containers to be both blast resistant to explosive devices that are below the detectable threshold and are also light in weight. It is desirable that containers for use on ships and trucks are also light in weight and they must be made explosion proof for the same reasons as for airplanes particularly if they are carrying hazardous cargos.
From studies which have been conducted to determine how a standard ULD will react to an internal explosion, it is known that the panels which form the container of the ULD will tend to bulge outwardly from the effect of a blast deep within the baggage. However, if the source of the explosion is close to a panel, the panel will be easily ruptured. Furthermore, it is known that panels are relatively strong in structurally resisting the tensile stresses generated by the forces involved in normally loading baggage and other cargo carried in the intended design manner, but the panels are massively overmatched by a proximate explosive blast. That is, the panels are highly ineffective in resisting rupture from an explosive event. If the container is to survive a high blast loading, panels of significantly increased strength must replace them. Even after considerably increasing the resistance of the panels to blast, stress analysis shows that the highest stress concentrations that result from an explosion within the ULD occur at the points and around the door or hatch which covers the opening into the ULD. This gives rise to the situation where the panels survive but the joints split. One obvious means for providing a hardened ULD is to simply add more material at the points where the highest stress concentrations occur. It is preferable, however, to avoid this additional weight.
One way to strengthen ULDs and other containers to make them more resistant to blasts is to provide a hardened load carrying device for use in transporting cargo on aircraft, trucks, or ships, which hardened load carrying device is able to resist internal blasts without rupturing, particularly by incorporating reinforcing material at the points where an internal explosion generates the highest stress concentrations in the device. Examples of this are shown in Mlakar et al., U.S. Pat. Nos. 5,599,082, and 5,312,182 and Mlakar, U.S. Pat. Nos. 5,595,431 and 5,413,410.
Other experts in explosives and transport-survivability techniques have studied additional ways to make commercial airliners more resistant to terrorist bombs. One result of these studies has been the development and deployment of new generations of explosive detection devices. As a practical matter, however, there remains a threshold bomb size above which detection is relatively easy but below which an increasing fraction of bombs will go undetected. An undetected bomb would likely find its way into luggage, either carried on board (in cabin) by a passenger or stored in an aircraft cargo container.
Ashley, S., in Mechanical Engineering 114(6):81086, 1992, describes a number of redesigned aircraft cargo containers. One type of container described by Ashley is designed to suppress shock waves and contain exploding fragments by safely bleeding off or venting high-pressure gases. Another type of container is designed to guide explosive products overboard by channeling blast forces out of and away from the airplane hull. Several of these containers use composite materials that are both strong and light in weight. However, most of these hardened containers are too expensive and heavy for service with the airlines.
Explosive devices produce high velocity fragmentation emanating both from the device casing and from material close to the point of explosion, so-called secondary fragmentation. In addition, explosive devices produce shock waves that can be characterized by having a rise time that is a virtual discontinuity in the physical properties of the material through which it propagates. These shock waves produce the potentially highly damaging phenomenon known as blast. Shock waves travel at a speed related to their amplitude, with higher pressures traveling faster than lower pressures, and the characteristics of the given medium. Once produced, the shock wave propagates outwardly from the source of the explosion, obeying well-understood physical laws. These laws, the conservation of mass, momentum, and energy describe how the shock propagates through a medium and, importantly, how it propagates from medium to medium with the associated changes in velocity and pressure. Shocks propagating spherically away from the source of the explosion will drop in pressure very rapidly. The decay in pressure generated within or close to structures is highly dependant on the geometry surrounding the explosion. Reflective barriers, tunnels, corners, and many other structural features can reduce the rate at which the shock wave decays, and, in some circumstances, locally increase pressure.